1. Field of the Invention
The present invention relates to a laminate sheet material punching method, in which a laminate sheet material including a cover sheet is punched to form the cover sheet to be adhered to a disk substrate into a required planar shape. The present invention also relates to an optical disk manufacturing method, in which a transparent cover layer is formed by adhering the cover sheet onto the disk substrate.
2. Description of the Related Art
Optical disks for recording or playback information by using a laser beam, such as a Compact Disk-Recordable (CD-R), a Compact Disk (CD), a Digital Versatile Disk (DVD) and a Digital Versatile Disk-Recordable (DVD-R), are popular. In recent years, there is a demand for the optical disk to store a larger quantity of information such as image information, and therefore, the study of higher density has been promoted. The recording density of the optical disk generally depends on a spot size of a light beam on the disk. The spot size is proportional to λ/NA, wherein λ represents the wavelength of a laser beam, and NA represents the numerical apertures of an objective lens. Therefore, in order to enhance the recording density with respect to the optical disk, it is necessary to shorten the wavelength of the laser beam and increase the number NA of the objective lens. However, since coma aberration generated by the inclination of the optical disk is increased in proportion to the third power of NA, a margin with respect to the inclination of the optical disk becomes extremely small due to the increase of the number of NA, thereby blurring the beam spot even with a slight inclination so as to make it impossible to achieve the recording/playback in high density. For example, as disclosed in Japanese Patent Application Laid-open (JP-A) No. 11-31337, a cover layer, through which the laser beam passes, is formed in a sufficient thinness (for example, about 0.1 mm) in the optical disk which can cope with the high density, and thus, it is necessary to suppress an increase in coma aberration caused by the inclination of the disk in association with the increase of the number of NA.
In a process for manufacturing the above-described optical disk, for example, a thin film-like cover sheet formed of a transparent film as a basic material made of polycarbonate or the like is adhered to a recording surface side of a disk substrate having an information recording layer formed thereon, and by this cover sheet, a transparent cover layer is formed on the disk substrate. At this time, an adhesive film is previously formed on one of the side of the cover sheet, i.e., on a adhering surface, and the cover sheet is adhered to the recording side of the disk substrate via the adhesive film.
In many cases, the above-described cover sheet, which is laminated with a disk substrate, is manufactured by using a laminate sheet material of a four-layered structure, in which strip-off sheets and protective sheets are adhered to both of the adhesive surface and light incident surface of the elongated belt-like cover sheet, as a process material. In particular, for example, the protective sheet and the cover sheet in the laminate sheet material are punched in a circular or annular shape corresponding to the shape of the recording side of the disk substrate from the surface on the side of the protective sheet by a punching blade whose blade tip extends in an annular manner, and then, the punched cover sheet is adhered to the recording side of the disk substrate. At this time, the strip-off sheet adhered to the adhesive film of the cover sheet is not punched by the punching blade, but usually, remains in an elongated belt-like manner. The elongated belt-like strip-off sheet is used as a carrier base for conveying the cover sheet punched from the laminate sheet material. That is, the cover sheet per se is very thin (for example, about 80 to 100 μm), and therefore, it is difficult to convey the cover sheet without any occurrence of a defect such as creases. For this reason, conveying force is transmitted to the cover sheet via the strip-off sheet in the state in which the strip-off sheet remains on the cover sheet, thereby making it possible to readily convey the cover sheet punched in the planar shape corresponding to the shape of the recording side of the disk substrate without any occurrence of a defect such as creases.
FIG. 7 shows one example of a method for conveying a cover sheet punched from a laminate sheet material by a conventional punching method. In FIG. 7, a case in which a cover sheet 114 is conveyed onto a receiving table 112 while a strip-off sheet 118 is being stripped from the cover sheet 114 by a sheet strip-off apparatus 110 is shown. The sheet strip-off apparatus 110 is provided with a wedge-shaped strip-off guide member 111 for stripping off the strip-off sheet 118 from the cover sheet 114. The strip-off guide member 111 is supported in so that the tip portion thereof is oriented to a sheet conveying table.
Furthermore, the sheet strip-off apparatus 110 is provided with a tension mechanism (not shown) for applying tensile force to the strip-off sheet 118 in a predetermined strip-off direction (i.e., the direction indicated by arrow E) while applying predetermined tensile force to the strip-off sheet 118 having the plurality of cover sheets 114 adhered thereon. Consequently, in the sheet strip-off apparatus 110, the cover sheet 114 is moved from the base and of the strip-off guide member 111 toward the tip and thereof while being brought into press-contact with the strip-off guide member 111 via the strip-off sheet 118 by a constant press-contact force. At this time, the strip-off sheet 118 adhered to the cover sheet 114 is peeled off from the cover sheet 114 in the vicinity of the tip end portion of the strip-off guide member 111. Moreover, the cover sheet 114 from which the strip-off sheet 118 has been peeled off is pushed forward of the strip-off guide member 111, and slides onto the receiving table 112.
However, when the cover sheet is punched from the side of a protective sheet 120 of the laminate sheet material by a punching blade with the strip-off sheet 118 remaining thereon as a carrier base in an elongated belt-like manner, a substantially wedge-shaped burr 122, which projects toward an adhesive film 116 may be produced at the outer peripheral edge of a resin film 115, which forms the base material of the cover sheet 114, as shown in FIG. 8. The burr 122 produced at the resin film 115 degrades the adhesiveness between a disk substrate 117 and the outer peripheral portion of the cover sheet 114. Additionally, air may intrude between the disk substrate 117 and the cover sheet 114 in an optical disk or a warp may be generated at a light incident surface, thereby leading to a decrease in yield of a non-defective optical disk.